Sunday night was one of those situations that Danny Ainge prepared for this summer by stocking the Celtics bench with talent and youth.

It was Boston’s seventh game in 10 nights, the equivalent of the final day of double-time for the warehouse employee. It was going to be a grind, especially for 30-something starters Paul Pierce, Kevin Garnett, and Jason Terry.

The problem was, the youngsters (besides rookie Jared Sullinger) and depth players followed suit and played just as listless and exhausted. Ainge, the team’s president of basketball operations, signed Jeff Green to a long-term deal along with Courtney Lee and Leandro Barbosa to supplement the starters.

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This was supposed to be one of those times when those three shined, taking the responsibility and onus for one night from the Hall of Famers. This was an important game in the grand scheme. The Celtics would much rather be 7-4 entering a two-game stretch with Western Conference powers San Antonio and Oklahoma City than 6-5, so while losing likely won’t derail their short-term plans, it was costly.

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If the Celtics are going to be elite, they have to play elite more often. Blame fatigue or the heavy travel, but the Celtics lost to a Pistons team that entered 1-9 and had allowed 100 or more points in six games. The Celtics lost by 20 points (103-83) to a team whose coach — Lawrence Frank — may already be on the hot seat.

While the loss wasn’t devastating, it should not be dismissed. Boston had major troubles over the past few years with the second game of back-to-back sets, especially on the road. Age is a valid issue.

But the Celtics added younger players who shouldn’t be affected as much by heavy workloads. And yet the only Celtic who played with any passion was Sullinger, who led the team with 16 points in 27 minutes playing in front of family. Lee attempted two shots in 20 minutes. Green took five in 22 minutes. Barbosa, as can be expected, had no problem shooting but he missed five of his six attempts.

Fatigue because of age was an issue Ainge had been trying to avoid. That’s why replacing 37-year-old Ray Allen with 27-year-old Lee was supposed to inject enthusiasm. The impact of playing consecutive games and seven games in 10 days is legitimate. Games are grinds and the Celtics had no legs in Sunday’s loss.

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But that’s when bench players, who play fewer minutes and seek opportunities to make an impact, have to emerge. Green and Lee especially have to be more aggressive and take more responsibility.

Championship-caliber teams have off nights and the Celtics are allowed to play poorly, but Boston didn’t look like a physically retooled team Sunday. Coach Doc Rivers advised his players to dismiss the defeat, get some rest Monday, and prepare Tuesday for the Spurs with a light practice.

Meanwhile, an NBA source acknowledged that Darko Milicic may miss an undetermined amount of time because his mother is ill, and he may not return to the team. The Celtics are being very sensitive with Milicic and there isn’t likely to be movement unless he makes a decision to remain in his native Serbia.

The Celtics thought they were getting a potential steal in Milicic because of his defensive prowess and shot-blocking, but he has played just five minutes all season. If he remains in Serbia, the Celtics could pursue another post option that could help with the team’s rebounding woes.

Boston is 30th in rebounding and could use a boost. Kenyon Martin, the much-traveled No. 1 overall pick in 2000, is a free agent and hasn’t bolted for China or other opportunities like some of his aging brethren.

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Ainge signed Milicic and Jason Collins to add depth at center but neither is playing and that means Rivers doesn’t think they will be productive. If a roster spot opens, the Celtics should take a serious look at Martin.

The Celtics not only need big nights from the bench in blowout wins such as Saturday vs. Toronto, they need Green, Lee, or even Sullinger to assume control when the vets are weary. Ainge spent the offseason rebuilding the bench, making it younger and potentially more talented. And those additions can’t rely on the same excuses as the more accomplished Garnett, Pierce, Terry, and Rajon Rondo.